What is New Labour? It's a fresh, youthful leader in a crisp white shirt, evidently decent, engagingly optimistic and - even after many weeks of campaigning, still new. It is Millbank Tower, the blind-shaded symbol of sophisticated campaigning. It is Peter Mandelson's sphinx smile in the shadows. It is "traditional values in a modern setting".

It is, in sum, an opposition campaign, not a new philosophy or a blueprint for governing. Now, unless all our polled fellow citizens are compulsive, secretive liars, New Labour is on the verge of power. That means it will change. Whatever you thought New Labour meant, it is unlikely to mean in the future.

Up to now, New Labour has defined itself first and foremost as Not Old Labour. It isn't the closed shop. It isn't nationalisation. It isn't high taxes. It isn't incompetence, extremism, retreat. It isn't failure, failure, failure. Instead, it is an electable repudiation of the past. (And how.)

In government, however, Old Labour becomes a meaningless adversary. It is yesterday's enemy. Yes, there will be left-wing rebels. Yes, there will be trade union try-ons, upon which Tony Blair will trample easily and cheerfully. But there will not be a leftist economic and social programme, in opposition to which Blair can constantly define himself as new, fresh, different. How could there be? He himself has killed it.

In government, it isn't exciting or novel to be pro-business: it becomes a question of which businesses you are pro, and how. In government, prudent fiscal policy isn't something for interviewers and floating voters, but a painful and bruising monthly struggle. In government, being "patriotically pro-European" doesn't help you much when it comes to hard negotiating choices.

In government, the wizard campaigning and the tight central control of the Leader of the Opposition's Office become redundant. All those gleaming Millbank computers instantly become as useful as the last war's fighters. New Labour may have been a brilliant way of stopping the Conservatives getting away with it again. It has not yet become a way of governing.

What will happen when it does? One way of looking at the Blair project today is to compare it to the state of Margaret Thatcher's party and project in 1979. There are similarities: she came in, looking new and surprising (a woman, after all) during a wave of revulsion and boredom with the old Labour establishment. We did not know quite what she was up to. She was surrounded by shadowy thinkers and policy wonks.

Thatcher then had about as much respect for the Heath and Macmillan-era Tory party as Blair has for the late Eric Heffer. Like Blair, her ideas were not fully formed in 1979. Thatcherism evolved in power, as Margaret Thatcher's character and ideology seized the opportunities.

This Thatcher comparison is one that Blair's people quite like. Most of them grew up during the Thatcher era and it is hardly surprising, perhaps, that her memory is invoked as a model for strong leadership, if nothing else.

For that reason, it is worth analysing the lessons of Thatcher's radical Toryism. The big lesson is obvious - that successful governments need a clear sense of leadership and a deliverable agenda. Thatcher may not have known how she was going to liberalise, deregulate Britain, but she knew that was what she wanted to do.

There is no doubt who is in charge of New Labour. But what does it want to do? Actually, we know a lot. It has clear policy pledges on school class sizes; getting 250,000 young unemployed people into training and work; fast-track punishment for young criminals and improvements to the health service. These, alongside the constitutional promises, give the party's position a clarity that the Conservatives lacked in 1979.

This means we can judge Blair, at least partly, by whether he delivers on a limited number of plausible, realistic and specific promises. Given the cynicism about politics now, it is hard to overstate the possible importance of this. Imagine feeling that voting gave you a contract with government, which ministers then actually delivered. Imagine an election in 2002 during which we were able to compare his performance with his original guarantee.

This could be one way of reconnecting the unplugged democracy. Labour's gimmicky-looking pledge card could, if things worked out, be a kind of grand anti-cynicism campaign. If the classes were sorted out, the reforms accomplished and the health service recognisably improved, that would be a foolproof definition of success.

Blair's pledge list is a relatively modest one, and certainly less than a full five-year programme. What else would New Labour do? That will partly depend on what confronts it. The first Thatcher term, after all, was mostly shaped by the way in which she exploited events and challenges - stumbling on privatisation, smashing the miners, confronting the French and Germans over Britain's EC budget contribution.

There was a shape to her reactions because she had a project, a sense of direction. So what is the project, the bigger idea, to which Blair will be able to refer when crises (which are, in politics, opportunities) occur?

"Traditional values in a modern setting." Yesterday he called those "a fair deal, social justice". Excellent. But how do you keep pushing social justice forward when the windfall tax has been spent, and Gordon Brown is lashed to stern macro-economic positions inherited from the Conservatives, and it is an unforgiving world out there? After all, Blair's comfortable, Conran-sounding "modern setting" includes global pressure on tax rates and regulation; fierce competition between labour markets; and fast punishment for devaluers.

Part of the unspoken bargain voters will be making with Blair is that he, Brown, Robin Cook and the others will do their best; that when hard choices come, they will remember the bottom dog. That would be very welcome. It's what this country needs. But it will still be, in essence, a defensive response, far removed from the political blitzkrieg of Thatcherism.

Put it another way: she knew who her enemies were, and she knew she could beat them. A persistent niggle about New Labour is that it has no enemies, except for a few oafs at Conservative Central Office, and some reassuringly vague abstract nouns such as "pessimism" and "despair". Blair's embrace can seem implausibly wide: his is a coalition in which lions snuggle down with lambs, ancient foes are reconciled and a smooth surface of freshly laid optimism blankets old wounds.

I hope that part of the answer is that Labour would tackle excessive concentrations of power - at home, in Brussels, in the City and so on. That requires a willingness to pick fights with powerful players, real, rising forces in society. Blair's harshest critics will laugh. Yet he has been electioneering, not governing, and maybe different rules apply. Certainly, the Thatcher parallel reminds us of the importance of doing things in power. And that means making enemies.

There is one final aspect of the Thatcher comparison that is worth brooding on as the country waits to go to the polls. It could be called the character of government. People often forget that Margaret Thatcher came to power pledged to make government less oppressive; to pass power back down to local authorities; to smash up the quangos. She wasn't going to be a bully. And yet things did not quite turn out that way.

She was not a pluralist politician; she was a singular one. She developed a strong protective inner court which kept her away from a party of faint hearts and doubters. It made for a bunkered vision and eventually cut her off from the country as well. So when Blairites say they admire her style, it is perhaps reasonable for the rest of us to say: yes, but.

But where does it lead? Does it, for instance, imply that the current shadow cabinet in 1997 already contains Labour equivalents of the "wets" whom Thatcher sacked in the early 1980s in order to create her revolution? Are there people who will play the roles of Jim Prior, Peter Walker and Francis Pym?

Do we suppose that Blair will try to exercise absolutist control on his ministers and their departments - that the Millbank electoral campaign will be a dry run for government? Will the press be thumped with Ingham- like abandon?

That would be a sad and bad error of political style. This country needs a new leadership with more openness and generosity. We need a real fresh start - not another dark and petty Renaissance court perched atop an electoral landslide. A political style based on Thatcher might hold the attention of The Sun for a time but would repel the intellectual energy and real enthusiasm that any centre-left project requires.

So what is New Labour? A proposition, that's all, a sketch of a better future, a chance to start afresh. It is a changed party that is about to change the country.

How? New Labour will either prove to be a new kind of populist-conservative government, offering a slightly better deal to the users of state schools and hospitals, cracking down on the thugs, and running the country in a highly centralised, conventional way. The tone of Labour's campaigning would suggest that this is the likely outcome. The alternative outcome is the same ... except that New Labour is also politically radical, liberating and pluralist - that it escapes from the old grey philistinism of British politics, takes on some of the corporate baronies, reforms the voting system and fights to democratise the EU.

In the end, it's a question for that smiling, white-shirted newcomer. A simple one: how big is his ambition?